The present invention relates to biocompatible and bioabsorbable glass fibers for use in the area of medical implants, particularly orthopaedic and dental implants. More particularly, it relates to spun or drawn fibers containing primarily calcium oxide (CaO) and phosphorous pentoxide (P.sub.2 O.sub.5) which may be used as a reinforcement for resorbable polymeric bone plates and artificial ligaments.
Many different materials have been used in the area of orthopaedic and dental implant devices. For example, various metals have been used as internal fixation devices, the most common being compression plates, rods or pins. Today the vast majority of these devices are made up of stainless steel or titanium. They are used to hold fractured pieces of long bones in correct alignment and to minimize movement until a satisfactory union can take place by natural bone fracture healing processes.
This technique can provide quite satisfactory results; however, several problems can occur due to large differences in the modulus of elasticity of the bone and metal which usually results in the need for a second operation to remove the device. Also, for reasons not completely understood, some individuals do not tolerate the metal and the bone, or surrounding soft tissue, has an inflammatory response to the metal.
The problem of differing elastic modulus between the bone and metal plate and the need for a second operation can be, in some instances, directly related. In many cases, the person receiving the implant does not have the implant removed, either due to their own neglect or the surgeons. Even if a satisfactory bone union takes place, the large difference in the strength and modulus will actually result in a deterioration of the bone in the vicinity of the metal plate. If the condition is allowed to persist, severe pain and/or refracturing of the bone can occur. Also, if the plate is removed, the underlying healed bone (callus) is usually not well organized. That is, the composite structure of the bone has not aligned itself such that it has maximum strength along the direction (axis) of highest stress, which is the natural behavior of healthy mature bone. This can best take place if the stresses placed upon the bone are gradual, allowing the bone sufficient time to reorganize.
A similar problem exists in the attempted repair of torn, or partially torn ligaments or tendons. Ligaments are composed of interwoven strands of collagen fibers very much like hemp rope. When these fibers are broken, they will, if properly stabilized and the blood supply is adequate, repair themselves. In a manner similar to bone, the collagen fibers do not realign themselves along lines of maximum stress if the stresses are being distributed over a permanently placed artificial support, such as metal wires. They remain in a disorganized mass, like scar tissue, with a much reduced strength when compared to normal ligamentous material. If the supporting artificial material is surgically removed, if that is even possible, there is great danger of the patient suddenly applying stress to the ligament that is too high for it to withstand and further damage will occur.
For this reason, it has been proposed that a biocompatible composition be used in the repair or replacement of human body parts such as ligaments, tendons and bones. Thus, Alexander et al. in U.S. Pat. Nos. 4,329,743 and 4,411,027 disclose using a composite of a bioabsorbable polymer (polylactic acid) on a substrate of a plurality of carbon fibers. See also U.S. Pat. Nos. 4,141,087, 4,140,678 and 4,052,988 which diclose various bioabsorbable synthetic polymers for use as permanent sutures, artificial ligaments and bone plates. However, while the polymer of Alexander et al. is bioabsorbable, the carbon fiber reinforcement is not. Since 30-60% of the bone plates and artificial ligaments of Alexander et al. are made up of the carbon fibers, the problem of bioincompatibility remains due to their being stiff and non-bioabsorbable.
It is also known that bioabsorbable ceramic materials may be used as medical implants. See, for example, U.S. Pat. No. 4,218,355 (assigned to the same assignee as the present invention), which discloses use of an aluminum oxide/calcium oxide/phosphorous pentoxide porous ceramic material as a carrier for controlled release of proteins, polypeptide hormones and other substances. It is also known that this type of ceramic composition may be used as a bioabsorbable bone graft material. See also, U.S. Pat. No. 4,155,124, which discloses a burnt ceramic bone implant, and the IADR Abstracts in the March 1982 issue of the Journal of Dental Research, various ones of which disclose work with biodegradable ceramic materials.
Despite the recent efforts to develop a bioabsorbable composition or composite which may be used as an orthopaedic and dental implant, much work remains to be done. The bioabsorbable synthetic polymers do not by themselves have sufficient structural strength to be used alone as an implant and the porous ceramic materials tend to be too rigid and brittle. Accordingly, the need exists for an improved biocompatible and bioabsorbable material for use in the area of orthopaedic and dental implants, and particularly a material which can be used to reinforce existing bioabsorbable synthetic polymers to form a composite suitable for that purpose.